// now set to half the size, so we have 2x pixels for the image n.size = newNSSizeMBS(n.width/2, n.height/2)

// and returnReturn nEndFunction

Notes:
The thing to do is to have 2x the pixels, but assign a size to the image which gives it the right size in points.
You can pass the NSImageMBS from here to NSMenuItemMBS. For Retina displays, the full resolution is used. For others it will be reduced.